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          <div>
            <h2 class="title"><a id="fwrkmasterreplica"></a>Chapter 4. Replica versus Master Processes</h2>
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      <div class="toc">
        <p>
          <b>Table of Contents</b>
        </p>
        <dl>
          <dt>
            <span class="sect1">
              <a href="fwrkmasterreplica.html#determinestate">Determining State</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="processingloop.html">Processing Loop</a>
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          <dt>
            <span class="sect1">
              <a href="exampledoloop.html">Example Processing Loop</a>
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            <dl>
              <dt>
                <span class="sect2">
                  <a href="exampledoloop.html#runningit">Running It</a>
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      <p>
                  Every environment participating in a replicated application
                  must know whether it is a <span class="emphasis"><em>master</em></span> or
                  <span class="emphasis"><em>replica</em></span>. The reason for this is
                  because, simply, the master can modify the database while
                  replicas cannot. As a result, not only will you open
                  databases differently depended on whether the environment is
                  running as a master, but the environment will frequently
                  behave quite a bit differently depending on whether it
                  thinks it is operating as the read/write interface for
                  your database.
          </p>
      <p>
                  Moreover, an environment must also be capable of
                  gracefully switching between master and replica states.
                  This means that the environment must be able to detect when
                  it has switched states.
          </p>
      <p>
                  Not surprisingly, a large part of your application's code
                  will be tied up in knowing which state a given
                  environment is in and then in the logic of how to behave depending on
                  its state.
          </p>
      <p>
                  This chapter shows you how to determine your environment's
                  state, and it then shows you some sample code on how
                  an application might behave depending on whether it is a
                  master or a replica in a replicated application.
          </p>
      <div class="sect1" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h2 class="title" style="clear: both"><a id="determinestate"></a>Determining State</h2>
            </div>
          </div>
        </div>
        <p>
                          In order to determine whether your code is
                          running as a master or a replica, you implement a
                          callback whose function it is to respond to
                          events that happen within the DB library. 
                          Note that these events are raised whenever the state is established. 
                          For example, when the current environment becomes a client —
                          including at application startup — the
                          <code class="literal">DB_EVENT_REP_CLIENT</code> event is raised. Also, when an
                          election is held and a replica is elected to be a master, the
                          <code class="literal">DB_EVENT_REP_MASTER</code> event is raised on the newly elected master and the
                          <code class="literal">DB_EVENT_REP_NEWMASTER</code> is raised on the other replicas.
                  </p>
        <p>
                          Note that this callback is usable for events beyond
                          those required for replication purposes. In this
                          section, however, we only discuss the
                          replication-specific events.
                  </p>
        <p>
                          The callback is required to determine
                          which event has been passed to it, and then take
                          action depending on the event. For replication,
                          the events that we care about are:
                  </p>
        <p>
                      Some of the more commonly handled events are
                      described below. For a complete list of events, see
                      the

                      
                      <span>
                          <code class="methodname">DbEnv::set_event_notify()</code>
                          method in the <em class="citetitle">Berkeley DB C++ API Reference Guide</em>.
                      </span>
                      
                  </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                                      <code class="literal">DB_EVENT_REP_CLIENT</code>
                                      
                              </p>
              <p>
                                      The local environment is now a replica.
                              </p>
            </li>
            <li>
              <p>
                              <code class="literal">DB_EVENT_REP_CONNECT_BROKEN</code>
                              
                          </p>
              <p>
                              A previously established connection between
                              two sites in the replication group has been
                              broken.
                          </p>
            </li>
            <li>
              <p>
                              <code class="literal">DB_EVENT_REP_CONNECT_ESTD</code>
                              
                          </p>
              <p>
                              A connection has been established between
                              two sites in the replication group.
                          </p>
            </li>
            <li>
              <p>
                              <code class="literal">DB_EVENT_REP_CONNECT_RETRY_ESTABLISHED</code>
                              
                          </p>
              <p>
                              An attempt was made to establish a connection
                              to a known remote site, but the connection
                              attempt failed.
                          </p>
            </li>
            <li>
              <p>
                              <code class="literal">DB_EVENT_REP_DUPMASTER</code>
                              
                          </p>
              <p>
                              A duplicate master has been discovered in the
                              replication group.
                          </p>
            </li>
            <li>
              <p>
                              <code class="literal">DB_EVENT_REP_ELECTED</code>
                              
                          </p>
              <p>
                              The local site has just won an election and
                              is now the master. Your code should now
                              reconfigure itself to operation as a master
                              site.
                          </p>
            </li>
            <li>
              <p>
                              <code class="literal">DB_EVENT_REP_ELECTION_FAILED</code>
                              
                          </p>
              <p>
                              The local site's attempt to initiate or
                              participate in a replication master election
                              failed, due to the lack of timely message
                              response from a sufficient number of remote
                              sites. 
                          </p>
            </li>
            <li>
              <p>
                              <code class="literal">DB_EVENT_REP_ELECTION_STARTED</code>
                          </p>
              <p>
                              Replication Manager has started an election to choose a master site.
                          </p>
            </li>
            <li>
              <p>
                              <code class="literal">DB_EVENT_REP_LOCAL_SITE_REMOVED</code>
                              
                          </p>
              <p>
                                  The local site has been removed from the group.
                          </p>
            </li>
            <li>
              <p>
                                      <code class="literal">DB_EVENT_REP_NEWMASTER</code>
                                      
                              </p>
              <p>
                                      An election was held and a new environment was made a
                                      master.  However, the current environment <span class="emphasis"><em>is
                                          not</em></span> the master. This event exists so that
                                      you can cause your code to take some unique action in the
                                      event that the replication groups switches masters.
                              </p>
            </li>
            <li>
              <p>
                                  <code class="literal">DB_EVENT_REP_MASTER</code>
                                  
                              </p>
              <p>
                                      The local environment is now a master.
                              </p>
            </li>
            <li>
              <p>
                              <code class="literal">DB_EVENT_REP_MASTER_FAILURE</code>
                              
                          </p>
              <p>
                              The connection to the remote master replication site has failed. 
                          </p>
            </li>
            <li>
              <p>
                              <code class="literal">DB_EVENT_REP_PERM_FAILED</code>
                              
                          </p>
              <p>
                              The Replication Manager did not receive enough acknowledgements to
                              ensure the transaction's durability within the replicationg group.
                              The Replication Manager has therefore flushed the transaction to
                              the master's local disk for storage.
                          </p>
              <p>
                              How the Replication Manager knows whether the acknowledgements it
                              has received is determined by the ack policy you have set for your
                              applicaton. See <a class="xref" href="fwrkpermmessage.html#fmwrkpermpolicy" title="Identifying Permanent Message Policies">Identifying Permanent Message Policies</a>
                              for more information.
                          </p>
            </li>
            <li>
              <p>
                              <code class="literal">DB_EVENT_REP_SITE_ADDED</code>
                              
                          </p>
              <p>
                              A new site has joined the replication group.
                          </p>
            </li>
            <li>
              <p>
                              <code class="literal">DB_EVENT_REP_SITE_REMOVED</code>
                              
                          </p>
              <p>
                              An existing site has been removed from the replication group.
                          </p>
            </li>
            <li>
              <p>
                                      <code class="literal">DB_EVENT_REP_STARTUPDONE</code>
                                      
                              </p>
              <p>
                                      The replica has completed startup
                                      synchronization and is now processing
                                      log records received from the master.
                              </p>
            </li>
            <li>
              <p>
                              <code class="literal">DB_EVENT_WRITE_FAILED</code>
                              
                          </p>
              <p>
                              A Berkeley DB write to stable storage failed.
                          </p>
            </li>
          </ul>
        </div>
        <p>
                          Note that these events are raised whenever the
                          state is established. That is, when the current
                          environment becomes a replica, and that includes
                          at application startup, the event is raised.
                          Also, when an election is held and a replica is elected to be a
                          master, then the event occurs.
                  </p>
        <p>
                          The implementation of this callback is fairly
                          simple. First you pass a structure to the
                          environment handle that you can use to record the
                          environment's state, and then you implement a switch
                          statement within the callback that you use to
                          record the current state, depending on the
                          arriving event.
                  </p>
        <p>
                          For example:
                  </p>
        <pre class="programlisting">#include &lt;db_cxx.h&gt;
/* Forward declaration */
void *event_callback(DbEnv *, u_int32_t, void *);

...

/* The structure we use to track our environment's state */
typedef struct {
    int is_master;
} APP_DATA;

...

/*
 * Inside our main() function, we declare an APP_DATA variable.
 */
APP_DATA my_app_data;
my_app_data.is_master = 0; /* Assume we start as a replica */

...

/*
 * Now we open our environment handle and set the APP_DATA structure
 * to it's app_private member.
 */
DbEnv *dbenv = new DbEnv(0);
dbenv-&gt;set_app_private(&amp;my_app_data);

/* Having done that, register the callback with the
 * Berkeley DB library
 */
dbenv-&gt;set_event_notify(event_callback); </pre>
        <p>
        That done, we still need to implement the callback itself. This
        implementation can be fairly trivial. 
</p>
        <pre class="programlisting">/*
 * A callback used to determine whether the local environment is a 
 * replica or a master. This is called by the Replication Manager
 * when the local environment changes state.
 */
void *
event_callback(DbEnv *dbenv, u_int32_t which, void *info)
{
    APP_DATA *app = dbenv-&gt;get_app_private();

    info = NULL;                /* Currently unused. */

    switch (which) {
    case DB_EVENT_REP_MASTER:
        app-&gt;is_master = 1;
        break;

    case DB_EVENT_REP_CLIENT:
        app-&gt;is_master = 0;
        break;

    case DB_EVENT_REP_STARTUPDONE: /* fallthrough */
    case DB_EVENT_REP_NEWMASTER:
        /* Ignore. */
        break;

    default:
        dbenv-&gt;errx(dbenv, "ignoring event %d", which);
    }
} </pre>
        <p>
        Notice how we access the
        <code class="literal">APP_DATA</code> information using the environment
        handle's <code class="literal">app_private</code> data member. We also ignore
        the <code class="literal">DB_EVENT_REP_NEWMASTER</code> and
        <code class="literal">DB_EVENT_REP_STARTUPDONE</code> cases since these are not
        relevant for simple replicated applications.
</p>
        <p>
        Of course, this only gives us the current state of the environment. We
        still need the code that determines what to do when the environment 
        changes state and how to behave depending on the state (described
        in the next section).
</p>
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